1. Field of the Invention
The present invention relates to an objective lens used for an optical pickup apparatus for an information recording and reproducing apparatus capable of performing at least one of the recording of information onto an optical information recording medium and the reproducing of the information recorded on the optical information recording medium, and to an optical pickup apparatus equipped with the objective lens. In particular, the present invention relates to an objective lens to condense a plurality of light fluxes including at least a light flux of a wavelength within a range from 380 nm to 420 nm and a light flux of a wavelength within a range from 630 nm to 810 nm onto an optical information recording medium, and to an optical pickup apparatus equipped with the objective lens.
2. Description of Related Art
Conventionally, an optical pickup apparatus (information recording and reproducing apparatus) has been used as an apparatus to perform the recording of information onto an optical information recording medium and the reproducing of the information recorded on an optical information recording medium. The optical pickup apparatus condenses a light emitted from a semiconductor laser light source onto the information recording surface of an optical information recording medium with an objective lens (optical lens) to perform the recording and the reproducing of information.
Moreover, in recent years, the following has been performed. That is, the trials of miniaturizing the diameter of a light beam spot, that is to say, the trials of narrowing down a beam spot sufficiently, with an objective lens so as to enable the recording and the reproducing of information with a light in a high-density recording state in order to enlarge the capacity of an optical information recording medium. Because the diameter of a beam spot is in inverse proportion to the numerical aperture (NA) of an objective lens, the enlargement of the NA of the objective lens has progressed, and an optical pickup apparatus has been recently developed that condenses a light flux of the wavelength in the range from 380 nm to 420 nm onto an optical information recording medium (called as a Blu-Ray Disc) with an objective lens having a numerical aperture on the side of a light source within a range from 0.8 to 0.9.
Moreover, it is insufficient for the optical pickup apparatus to be able to perform the recording and/or the reproducing of just one kind of optical information recording medium, and it is required for the optical pickup apparatus to deal with optical information recording media, such as a conventional compact disk (CD) (for the wavelength of about 780 nm, and for an NA of 0.45) and a digital versatile disk (DVD) (for the wavelength of about 650 nm, and for an NA of 0.6).
Now, devices for increasing the transmittance of optical components of an optical pickup apparatus have been performed to the optical components in order to use a light flux emitted from a light source efficiently. For example, an antireflective film has been formed on an optical surface of an objective lens or the like to suppress the quantity of lights reflected from the optical surface by the use of the interference of lights.
For example, a technique disclosed in Japanese Patent Application No. 2007-127954 forms one to three layers of antireflective films on the light source side surface of an objective lens used for a Blu-ray dedicated optical pickup apparatus having a numerical aperture (NA) of 0.85 on the light source side, and thereby suppresses the quantity of reflected lights to increase the transmittance of the objective lens.
However, when the antireflective films are formed on an objective lens used for a compatible optical pickup apparatus which can read and/or write information with a plurality of wavelengths, it has been difficult for the technique disclosed in Japanese Patent Application No. 2007-127954 to realize anti-reflection to each of the incident light fluxes having different wavelengths.
Although brief description of the above technique is given here, Japanese Patent Application No. 2007-127954 is not yet a laid-open publication at the time of the filing of the present application. Further, the inventor and the assignee of Japanese Patent Application No. 2007-127954 are the same as those of the present application. Thus, note that the inventor does not regard Japanese Patent Application No. 2007-127954 as a prior art of the present application.
On the other hand, it is known that the larger the incident angle of a light is, the more the wavelength dependency of the reflectance of the light shifts toward shorter wavelengths in the anti-reflection coat. Moreover, the more the light enters outer circumferential part of an objective lens from the central part thereof, the larger the incident angle of a light is. Consequently, in the conventional objective lens, the wavelength dependency of the reflectance of a light in a circumferential part shifts toward a shorter wavelength more than that of a light in the central part. As a result, the wavelength at which the reflectance of the light entering the circumferential part becomes the local minimum is shorter than that at which the reflectance of the light entering the central part becomes the local minimum. Consequently, in an objective lens provided with a conventional anti-reflection coat, the reflectance of a laser light has been low in the central part of the objective lens, but the reflectance of the laser light has been high in the circumferential part of the objective lens. Consequently, the transmitted light quantity in the circumferential part has become relatively smaller than that in the central part. As a result, the objective lens has had the problems of the deterioration of the spectral intensity of the transmitted lights of the whole lens, the enlargement of the diameter of a beam spot caused by the lowering of the converging performance of a light, the lowering of the light quantity of a beam, and the like.
Because the curvature of the lens surface of an objective lens having a high NA is large, the incident angle of a light in the circumferential part of the lens is very large. Consequently, the degree of the lowering of the quantity of a light transmitting the circumferential part of the objective lens becomes very large, and the enlargement of a spot diameter cannot be suppressed despite the use of the objective lens having the high NA. The situation like this is an obstacle to the enlargement of the capacity of an optical recording medium.
Japanese Patent Application Laid-Open Publication No. 2005-31361 discloses an objective lens securing a good balance between the miniaturization of the diameter of a spot and a transmitted light quantity by forming a specific antireflective film to set a local minimum reflected wavelength of an anti-reflection coat onto the longer wavelength side than the wavelengths within the range from 380 nm to 420 nm which are in use.
In order to obtain this sort of the objective lens, it is necessary to set the wavelength band in which the anti-reflection coat works to be wide. As a result, a maker is obliged to accept the enlargement of the number of the layers of the antireflective film, the thickness of each layer, and the total film thickness. However, the enlargement of the number of the layers, the thickness of each layer, and the total film thickness causes the problems of the lowering of the environment resistance to a high temperature environment and a high humidity environment, and of the lowering of the light resistance to prolonged light irradiation. However, Japanese Patent Application Laid-Open Publication No. 2005-31361 does not consider the balance between the transmitted light quantity of the objective lens, and the environment resistance and the light resistance. Thus the measures of Japanese Patent Application Laid-Open Publication No. 2005-31361 are insufficient.
Moreover, it became clear as a result of a further examination that the film thickness of an antireflective film decreased as the incident angle of a light into a lens enlarged, when the antireflective film was formed on the surface of the lens by the vacuum evaporation method. The optical film thickness at an incident angle θ was within a range from about D cos(3/4θ) nm to about D cos(8/9θ) nm to an optical film thickness D nm at an incident angle of 0°. It was also found that, because an evaporation source was placed at a position offset from the center of a dome on which the lens was set and the dome on which the lens was set had an arbitrary angle in a general evaporator, the optical film thickness was led to have a certain measure of distribution as the incident angle of the evaporation material from the evaporation source varied according to the direction and the position of the lens surface set on the dome. Consequently, also a problem became clear that, as the numerical aperture on the light source side became larger, the difference of the degrees of decreases of the film thicknesses became larger, and that the reflectance at a circumferential part of a lens became higher, which made the transmittance of the circumferential part fell further.